This invention relates to hydrostatic transaxles which are now used in increasing numbers for lawn care and other outdoor duties as the preferred choice for power transmission drive lines in products such as lawn and garden tractors, pedestrian walk-behind mowers and snow blowers.
Hydrostatic transaxles of the type currently sold in the marketplace require careful assembly and service practices in order to avoid certain problems occurring that may result in lower than expected operational life of the product. Hydrostatic transmissions operate most effectively and efficiently when they are constructed with exceedingly small clearances between their reciprocating and sliding elements. The transmission of power by such hydrostatic transmissions has now become widespread and the attendant small fluid leakage loss from the internal pressurized circuit during operation which is inherent with this type of speed changing device is generally considered insignificant as the resulting retardation in vehicle speed most often goes largely unnoticed by the end user. Hydrostatic transmissions work well and have a long and useful life so long as the level of contamination suspended in the power transmission fluid remains low. High levels of contamination carried by the power transmitting fluid can rapidly wear out the aforementioned fine clearances resulting in an increase in fluid leakage, especially during high pressure operation.
It is therefore of paramount importance during both the initial assembly process when the hydrostatic transaxle is built as well as at subsequent service repair intervals, that the possibility of contamination entering the hydrostatic transmission and its surrounding fluid chamber be minimised. In simple terms, the chance for the hydrostatic transmission components being contaminated during handling on the assembly lines in the factory recede as the number of components in the total assembly build is reduced. It follows therefore, that if the hydrostatic transmission could be fully assembled and sealed in the housing prior to the reduction gearing and differential shaft components being added, there would be an advantage.
In the past, because hydrostatic transmissions were of the self-contained xe2x80x9cbolt-onxe2x80x9d type designed specifically to be fitted to an interface provided on an exterior mounting face of a separate transaxle containing the reduction gear train, the possibility of contamination entering the hydrostatic transmission was not an issue when some form of a repair was needed in the separate transaxle drive train unit. For example, in the event that the hydrostatic transmission needed to be replaced, this type of repair could be quickly undertaken just by removing a few bolts in order to separate the unit from the transaxle and replace it with another unit. If on the other hand, a bearing or shaft seal needed to be replaced in the transaxle drive train, such a repair could be easily effected just by dis-assembly of the transaxle and without disturbing the internal components of the hydrostatic transmission which would stay in-place in their own housing. A typical design of the so-called xe2x80x9cbolt-onxe2x80x9d self-contained xe2x80x9cstand-alonexe2x80x9d type of hydrostatic unit for mating to a separate transaxle device is shown in Eaton Corporation U.S. Pat. No. 5,234,321 incorporated herein as reference.
Due to improvement in the art during the past decade or so, the vast majority of hydrostatic transmissions now in use are of the integrated type whereby a common housing is used to surround both the hydrostatic elements as well as the speed reducing gearing (and differential when required), typically as shown in FIGS. 3 through 5 in Thoma et al. U.S. Pat. No. 4,979,583 incorporated herein as reference. Although the improvement of the xe2x80x9cintegratedxe2x80x9d type over the earlier xe2x80x9cbolt-on stand-alonexe2x80x9d type of hydrostatic transmission and transaxle combination has provided significant economic benefits in terms of lowering manufacturing cost of the drive line such that hydrostatic transmissions are now better able to compete more effectively with mechanical-shift gear transmissions, inconveniences can arise when repairs are needed. The consequence of shipping units back to the factory for repair is both costly and inconvenient for the vehicle owner.
Furthermore, in the event of a service agent electing to make the repair himself, for example, a normally relatively simple repair involving the replacement of a worn bearing or seal, it is at present a fact that this would first necessitate the splitting open of the transaxle housing in order to gain access to those elements needing replacement. As such action results in the hydrostatic transmission components being exposed to what may well be a relatively unclean working environment, a distinct possibility exists that the hydrostatic transmission might have become contaminated such the repair is only short lived. Consequently, the service agent may elect to substitute the faulty unit with a brand new replacement but this has the disadvantage of much additional expense for the vehicle owner, especially if the existing hydrostatic transmission or conversely, the original gear train components were considered by the agent to be in good and still usable condition. There therefore is a need in the art for a new integrated hydrostatic transaxle that will allow simple repairs to be undertaken by the dealership on the non-hydrostatic components without exposing the internally disposed hydrostatic transmission components to contamination.
With all known integrated hydrostatic transaxles currently sold, factory testing can only take place once the transaxle is fully assembled as the hydrostatic portion as well as the geared portion are contained within a surrounding two-piece housing structure. In the event the factory test indicates that the hydrostatic transmission is not operating satisfactorily, repair and rectification can be both costly and time consuming as the complete housing must first be dismantled in order to be able to replace deficient hydrostatic componentry. What is therefore needed in the art a new form of integrated hydrostatic transaxle in which the two types of power transmitting componentry within the complete product package are separate from each other such that the hydrostatic transmission can be tested and approved before the remaining non-hydrostatic components are assembled in place. What is also needed is a new form of integrated hydrostatic transaxle allowing rectification work, when needed, to be speeded up and therefore more economic to perform. What is further needed is a new solution whereby the amount of handing required during assembly on the assembly lines is minimised before the hydrostatic transmission is fully sealed within the surrounding housing structure.
As integrated hydrostatic transaxles of the type currently available in the market require a large housing structure for containing both hydrostatic and non-hydrostatic components, the machine tools needed to perform finish machining operations on the housing are expensive due to their size. There would be a saving in machine tooling investment if the size of transaxle housing were smaller in size, and there would be further saving in terms of economies of scale if one part of the housing structure of the hydrostatic transaxle could be used for numerous other product types. What is therefore needed is a new form of integrated hydrostatic transaxle having a relatively small housing component requiring machining for the mounting of the hydrostatic transmission such that the remaining and larger housing members required for completion of the transaxle housing structure can be used in their as-received die-cast condition. What is further needed is a universal cover housing element for the mounting of the hydrostatic transmission such that the sub-assembly can be used in combination with any number of different case housing elements to satisfy a range of products types.
From one aspect the invention consists in a housing structure for a hydrostatic transaxle where the housing construction comprising three housing elements that inter-relate to form separate chambers for the hydrostatic transmission components and the geared components. An input shaft is supported in the housing and extends into the chamber containing the hydrostatic transmission to drive the hydraulic pump. An output shaft is also supported in the housing and extends into that chamber containing the geared components. In instances when a mechanical differential is also located within the chamber containing the geared components, the output shaft then comprises two shafts that extend from the differential in opposite directions. Within the chamber containing the geared components, the output shaft or shafts is drivingly engaged to the speed reduction gears and where the gears are driven by a connecting shaft that forms the power transmitting link between the hydraulic motor in the hydrostatic chamber and the geared components in the gear chamber.
By this invention, the hydrostatic transmission components for the hydrostatic transaxle can be assembled in a clean room and tested before the complete sub-assembly containing the hydrostatic transmission is dispatched to another location where the non-hydrostatic components are added. As the hydrostatic sub-assembly is sealed by the surrounding housing before entering the final assembly production lines, there is no chance for the hydrostatic transmission to become contaminated when the remaining components are added. In instances when the transaxle manufacturer elects to sub-contract the task of building the complete hydrostatic transmission to an outside agency, the supplied hydrostatic sub-assembly can be received in a ready-to-use condition thereby avoiding any need for the transaxle manufacturer to undertake inspection procedures to ascertain that the received goods are free from contamination from shipping and handling.
It is therefore an object of the invention to provide an improved housing for a hydrostatic transaxle whereby the chamber for the hydrostatic transmission components is segregated from that chamber containing the speed reducing geared components in a manner whereby the hydrostatic transmission can be tested and approved before the remaining assembly involving the geared components takes place. It is a further object of the invention to provide an improved housing for a hydrostatic transaxle whereby the service life of the unit can be extended by allowing simple repairs to be effected in the field without disturbance or disassembly of the hydrostatic transmission components.
What is further needed in the art is a new form of integrated hydrostatic transaxle in which a relatively small housing component structured for carrying the hydrostatic transmission be provided with an ability to resist and absorb within its structure the fluid pressure generated loads by the hydrostatic transmission such that the remaining and larger housing members of the hydrostatic transaxle serve to support the non-hydrostatic loads. It is a further object of the invention to group all the machining operations for the housing structure of the hydrostatic transaxle into said smallest of the three housing elements thereby providing material saving in tooling investment and total machining hours required.
It is a further object of the invention to segregate the hydrostatic transmission from the reduction gearing by providing a case housing element with a substantially planar and horizontally peripheral seam serving as an abutment surface onto which interface two smaller sized container-shaped housing elements whereby the power transmission link connecting the hydrostatic transmission to the reduction gearing has an axis of rotation arranged in parallel relationship with regard to the seam and where the housing elements serve to protect the power transmission link from corrosion or falling debris such as grass chippings which commonly accumulate on the exterior surface of hydrostatic transaxle apparatus.
In one form thereof, the hydrostatic transaxle of the invention comprises an axle assembly having a housing including first, second and third housing elements joined along a substantially planar and horizontally peripheral seam formed therebetween such that the first and second housing elements are to one side of the seam; the third housing element being provided with first and second cavities and where the first cavity is closed by an opposite cavity provided by the first housing element to define a first chamber. The second cavity is closed by an opposite cavity provided by the second housing element to define a second chamber; a hydrostatic transmission comprising a variable-displacement pump and fixed-displacement motor disposed within the first chamber and speed reducing gearing disposed within the second chamber; at least one outwardly extending output power transmission shaft rotatably mounted in the housing and an input power transmission shaft rotatably mounted in the housing and operatively connected to the pump, the hydraulic motor being operatively connected to the output power transmission shaft by means of the speed reducing gearing, and where a connecting shaft spans across from the first chamber to enter the second chambers to provide the power transmission link between the hydrostatic transmission and the speed reducing gearing.
Although the preferred shaft mounting location shown in this invention for the mechanical drive connection from the hydraulic motor to the projecting axle output is disposed along the seam whereby all three housing elements provide support surfaces in the form of semi-cylindrical pockets, an alternative embodiment is disclosed whereby such support surfaces are disposed fully to one side of the seam. An advantage of this alternative embodiment is that it allows at least one of the three housing elements to have a substantially flat profile at the peripheral seam, for instance the gear housing element, and where this housing element may manufactured in various materials such as an aluminium alloy casting; a pressed-steel component or as a simple plastic or nylon moulding.
In the embodiments described below, the fluid pressure generated loads by the hydrostatic transmission are easily absorbed and contained within the smallest of the three housing elements while the power transmission link between the hydraulic motor in the hydrostatic chamber and the geared components in the gear chamber provide hitherto unattainable improvements and savings in terms of assembly and repair practices over the integrated hydrostatic transaxles types presently on the market.